This invention relates to an apparatus for thermal processing of raw materials in meal form, particularly in the production of cement clinker from raw meal, having raw meal preheater, precalcination stage, rotary kiln and clinker cooler, according to the preamble of Claim 1.
In apparatuses for the production of cement clinker from cement raw meal, in order to avoid uneconomically long and/or large in diameter rotary kilns and to hold down the specific heat requirement of the cement clinker production process, it is known to connect upstream of the rotary kiln, as viewed in the material flow direction, a precalcination stage, which is equipped with at least one secondary firing (besides the firing in the rotary kiln). In a known precalcination stage, the rotary kiln off-gas riser supplied with fuel and preheated raw meal, or the gas-solids suspension contained therein is diverted by 90xc2x0 (U.S. Pat. No. 4,080,218) or by 180xc2x0 (EP-B-0 222 044) and admitted to the lowermost cyclone of the cyclone suspension preheater system in order to separate the precalcined raw meal from the gas stream. Along with the requirement of the highest possible quality precalcination of the cement raw meal before admission to the rotary kiln, the builders and operators of cement clinker production lines are being confronted at the same time with more stringent requirements as to low emission values for pollutants such as NOx and CO.
In the known precalcination stages, the rotary kiln off-gas is combined with at least one substream of tertiary air coming from the clinker cooler before admission to the lowermost cyclone of the cyclone suspension heat exchanger. Before this combining, the fuel admitted to the rotary kiln off-gas riser is combusted in richer than stoichiometric fashion, that is, with a deficiency of oxygen, in order to establish a CO-containing reduction zone or CO gas strands for the reduction of the pollutant NOx, which is formed particularly by high-temperature combustion in the rotary kiln (thermal NOx), while the fuel admitted to the adjacent tertiary air duct is combusted in leaner than stoichiometric fashion, that is, with an excess of oxygen (EP-B-0 222 044). The CO not consumed in the NOx reduction zone of the rotary kiln off-gas duct, as well as any solid fuel particles not initially combusted in the precalcination stage, are then, downstream as viewed in the direction of suspension flow, subjected to secondary combustion with excess oxygen from the tertiary air duct, this residual burnout being promoted by the gooseneck-shaped 180xc2x0 diversion of the suspension flow in the precalcination stage.
In the known precalcination stages, the effectiveness of the CO-containing reduction zone formed in the rotary kiln off-gas duct depends on the O2 content of the rotary kiln off-gas in relation to the quantity of fuel admitted via the at least one DeNOx burner. It is understood that this ratio cannot be accurately adjusted by control action on the induced-draft fan that is connected downstream, as viewed in the direction of off-gas flow, of the cyclone suspension heat exchanger system, because, aside from the rotary kiln off-gas, this induced-draft fan simultaneously takes in and exhausts the tertiary air as well. This ratio also cannot be exactly controlled by building an air choke into the tertiary air line in the precalcination stage before the confluence of the tertiary air line into the rotary kiln off-gas duct (DE-A-35 22 272, FIG. 1).
It has also been proposed already (e.g., DE-A-197 32 778, FIG. 6) that a baffle be built into the rotary kiln off-gas riser in a cement clinker production line below the raw meal inlet, which baffle, because of the severe mechanical/thermochemical stress it experiences, is subject to severe wear and therefore is designed as a plate-shaped hollow body through which coolant flows. Its cooling does prolong the service life of the known baffle; if, however, it were built into the CO-containing reduction zone or in the CO-containing gas strands of a rotary kiln off-gas duct equipped with DeNOxburner, it would introduce turbulence into the laminar character of the gas-solids suspension by vortex formation and thus in undesirable fashion shorten the length of the CO-containing reduction zone, shorten the residence times associated therewith, and ultimately impair the effectiveness of the NOx decomposition reaction.
In systems for cement clinker production of the above-described kind, having a precalcination stage connected upstream of the rotary kiln, in which precalcination stage a CO-containing reduction zone in the rotary kiln off-gas duct is employed to reduce the pollutant NOx, it is an object of the invention to make certain that the quantitative ratio of O2 in the rotary kiln off-gas to fuel admitted via DeNOx burner, and thus the effectiveness of NOx reduction, can be purposely controlled.
According to the invention, this object is achieved with an apparatus having the features of Claim 1. Advantageous developments of the invention are identified in the dependent Claims.
For the purpose of diminishing NOx in the rotary kiln off-gas in the cement clinker production line according to the invention, with the precalcination stage having at least one DeNOx burner with rich fuel combustion in the rotary kiln off-gas duct, the effectiveness of the CO-containing reduction zone is not only influenced indirectly by the adjustment of air chokes that can be built into the tertiary air lines coming from the clinker cooler before their confluence into the rotary kiln off-gas duct, and/or by the adjustment of the induced-draft fan that simultaneously exhausts the rotary kiln off-gas and the tertiary air, but according to the invention the fuel-to-oxygen ratio at the DeNOx burner and thus the effectiveness of the CO-containing reduction zone is directly controlled by building a throttle valve into the rotary kiln off-gas duct above the DeNOx burner and above the raw meal inlet but before or below the confluence of a tertiary air stream, which throttle valve directly controls the quantity of rotary kiln off-gas and thus the O2 content in the kiln off-gas and thus the CO content in the CO-containing reduction zone.
As the O2 content in the rotary kiln off-gas rises, the throttle valve built into the rotary kiln off-gas duct here is placed in pivoted positions that more strongly throttle the rotary kiln off-gas stream, and conversely. When the rotary kiln off-gas is throttled, its O2 content decreases, and if the quantity of DeNOx fuel is unchanged, the CO content in the CO-containing reduction zone increases, and conversely. Thus the O2 content in the rotary kiln off-gas, measured in suitable fashion in the rotary kiln inlet chamber, can lie in a range of roughly 1.0% to 3.0%, and the CO content in the CO-containing reduction zone and thus the efficiency of this zone can be controlled in comparatively accurate fashion.
The throttle valve built into the rotary kiln off-gas duct has its pivoting range in the flow direction of the off-gas, that is, the throttle valve articulation is arranged on the bottom side of the valve, on which the off-gas impinges. Within its pivoting range, the throttle valve thus stands perpendicularly to obliquely upwardly. In this way, the character of the flow of rotary kiln off-gas remains laminar in all pivoted positions of the throttle valve, and the length of the CO-containing reduction zone or, respectively, CO-containing strands of off-gas and thus also the residence time of the reactants in this CO-containing cloud remain as long as possible. Furthermore, disturbing material accretions due to flow separations at the throttle valve do not occur.
The service life of the throttle valve in the rotary kiln off-gas, which can have a temperature of approximately 800 to 900xc2x0 C. after the introduction of the raw meal to be calcined, is further increased if, according to a further feature of the invention, the throttle valve is designed as a plate-shaped hollow body through which a coolant, such as cooling air, cooling water, etc., flows.
The invention and its further features and advantages are explained in greater detail on the basis of exemplary embodiments illustrated schematically in the Figures.